metasomatoisen

Metasomatoisen is the Finnish adjective used to describe rocks or zones affected by metasomatism. Metasomatism is the chemical alteration of rocks through interaction with hydrothermal fluids, accompanied by substantial element transfer and formation of new minerals. The process contrasts with simple metamorphism by focusing on fluid-mediated chemical exchange rather than purely solid-state recrystallization.

Mechanisms and products: Fluids from magmatic, subduction-related, or devolatilization sources infiltrate rocks along fractures and pore spaces. Depending on temperature, pressure, and fluid composition, ions such as calcium, sodium, potassium, silicon, and metals are introduced or removed, producing minerals like calc-silicates (wollastonite, diopside), garnet, scapolite, albite, chlorite, calcite, and iron- or magnesium-rich phyllosilicates. Carbonate rocks commonly develop skarns or marble-like halos; ultramafic rocks may become serpentinites or listvenite.

Settings and significance: Metasomatism occurs in contact zones around intrusions, in regional alteration belts, and in ophiolitic and carbonate-rich terrains. It is important in ore genesis, forming skarn deposits rich in tungsten, tin, copper, or iron, and in creating economically valuable mineral assemblages. It also records fluid-rock interaction histories essential for interpreting tectonic evolution.

Study and evidence: Geological mapping, mineralogical analysis, and geochemical data, including isotope ratios and fluid-inclusion studies, help characterize metasomatism. In Finnish literature, metasomatoisen textures and mineral assemblages are described to distinguish metasomatic from primary igneous or metamorphic origins.

---